1. Field of the Invention
The present invention relates to electronic packaging. More particularly, the present invention relates to compression connectors, such as land grid array (LGA) connectors used to make electrical connection of modules to a printed wiring board.
2. Background and Related Art
Compression connectors, such as those employed in LGA connections and compression pad on pad (CPOP) type connectors, are well known in the art. Typical LGA compression connections employ an interposer to connect a multiple chip module (MCM) to a printed wiring board (PWB). The MCM generally comprises multiple integrated circuits or chips assembled into a subsystem the size of traditional single chip packages. The MCM is connected to the PWB through the interposer with the MCM circuit pads making contact with the array of connectors on one surface of the interposer and the PWB circuit pads making contact with the array of connectors on the other surface of the interposer. Compression forces are employed to hold the MCM and PWB against the interposer. Typical examples of such LGA compression connections can be seen by reference to U.S. Pat. Nos. 5,473,510 and 5,703,753.
Such compression connections may take a variety of particular packaging forms, such as shown in the above examples, but a common feature is to employ compressional force to hold the connectors and contact pads together. However, the typical compressional force required to provide reliable surface contact may, at times, create problems due to irregularities in surface profile of, for example, the PWB. More particularly, the compressional force required to provide good surface contact between mating pads and interposer connectors may further act to deform the PWB because of the irregularities in the surface profile of the non-mating surface of the PWB thereby compounding the irregularities and their effect.
It is known, that LGA interconnects also require relatively uniform surface contact between mating pads and LGA conductors across the array of pads to achieve reliable electrical connection. Accordingly, this necessitates not only a constant and uniform compressive force, but also some degree of conforming uniformity of the surface profile of the MCM and PWB. Thus, the uniformity of the thickness of the PWB, in particular, becomes a factor in achieving uniform compressive contact between mating pads and LGA conductors across the array.
It has been found, however, that the thickness of PWBs varies due to inherent design and manufacturing features. Compression-type connections in the PWB typically employ an array of Plated Through Holes (PTHs) with lands or pads around them as the contact surface for the interposer conductors to mate with. When the PTHs are made in a field or an array at the module site, the thickness of the PWB tends to be thinner in the region of the field or array of PTHs than in regions not having the PTHs. This is due to what may be called the xe2x80x9claminating effectxe2x80x9d.
The laminating effect is due to the fact that clearances in the power planes are required for isolation of the PTHs at various levels. Typically, such clearances require resin to fill in and form the power plane clearances as well as any other voids in the various layers of the PWB. However, the distance resin is able to flow is limited and does not flow through the complete module site to prevent reduction in thickness of the PWB in these PTH field or array regions. In addition, electroplated pads of a non-uniform plating thickness distribution may develop such that the outer area of the field or array of PTHs may be thicker than the center portion thereby compounding the laminating effect.
It has been found that this laminating effect forms gradual recesses or depression on opposing upper and lower surfaces of the PWB that can vary board thickness in the range of 2 to 7 mils, depending upon the PWB design and manufacture. However, the compliance of a typical LGA interposer, i.e., the amount of thickness variation the array of mating conductors thereon can effectively accommodate and still maintain good electrical contact, is around 2 to 4 mils. It should be understood that this thickness variation results in a depression occurring on each of the opposing surfaces of the PWB in range of 1 to 3.5 mils, and when compression of the PWB occurs, the upper depression of the PWB can deform into the lower depression forming an upper depression approximately two time that of its relaxed state.
Accordingly, it is an object of the present invention to provide an arrangement for thickness and surface profile control of laminate PWBs.
It is a further object of the present invention to provide an improved compression connection arrangement for LGA connection of electronic apparatus to a PWB.
It is a further object of the present invention to provide an improved connection method and structure for connecting a MCM to a PWB.
It is yet a further object of the present invention to provide an arrangement for thickness control for compression connectors used in electronic packaging.
It is still yet a further object of the present invention to provide a method and structure for thickness control of PWBs in compression connection of a module thereto.
It is another object of the present invention to provide an arrangement to compensate for thickness variations in a laminate PWB so as to provide improved electrical connection of a MCM in a LGA compression connection arrangement to the PWB.
In accordance with the present invention, thickness and surface profile variations in a PWB are controlled with the use of filler material. More particularly, thickness reduction in PWBs caused by fabrication processes in the region of electrical contact to an electronic module is corrected to provide effective electrical contact using compression connection techniques.
As stated above, the initial formation of PTHs and contact pads at the site of electronic module contact connection to a PWB act to cause a thickness reduction at the contact site as compared to the thickness of the surrounding region. The thickness reduction results in a gradual recess or depression on opposing upper and lower surfaces at the contact site of the PWB. Such depressions at the contact site cause difficulty in making good electrical contact between module and PWB using an interposer and compression connection techniques. The extent of the depression in the PWB is compounded when compressive forces are applied for the compression connection.
In accordance with the present invention, filler material, such as a bonding material, are employed between the PWB backside support and PWB at the site of the lower recess to correct for the thickness variation. Various amounts, types and shapes of filler material may be employed. Such material acts to reduce surface profile and thickness variation in the PWB and its concomitant effect of preventing relatively uniform and complete compression contact between the electronic module and PWB through an LGA interposer.